Optical networks utilize Reconfigurable Optical Add-Drop Multiplexers (ROADMs) to realize selective and reconfigurable add/drop of wavelengths or spectrum locally and between various degrees. ROADMs generally utilize Wavelength Selective Switches (WSSs) in different configurations. Flexibility in add/drop requirements has led to so-called colorless, directionless, and optionally contentionless add/drop multiplexer structures, such as in Reconfigurable Optical Add/Drop Multiplexer (ROADM) devices, nodes, architectures, and structures. A colorless add/drop device supports any wavelength or spectral occupancy/band being added to any port of an add/drop device, i.e., ports are not wavelength specific. A directionless add/drop device supports any port being directed to any degree. Finally, a contentionless add/drop device supports multiple instances of the same channel (wavelength) in the same device. A colorless, directionless add/drop device can be referred to as a CD device, and a colorless, directionless, and contentionless add/drop device can be referred to as a CDC device. In conventional CD or CDC ROADMs, channels/modems are directly attached to the mux/demux element which usually is a Multicast Switch (MCS) but could also be a Contentionless WSS (CWSS).
Although CWSS technology will improve scaling by offering higher port count and lower insertion loss compared to conventional MCS technology, it does not go far enough in terms of improving scaling and cost/port metrics when compared to other colorless ROADM architectures such as CDA (Colorless Direct Attach—a colorless structure which is not directionless or contentionless) and CD. For example, in a 16-degree CDC ROADM node built using state-of-the-art 1×32 WSSs coupled to 16×24 CWSSs, the maximum number of channels of local add/drop channels would be 384 (which is 16×24). Assuming 64 channels per degree, the total number of channels present at the node could be as high as 1024, so add/drop capacity, in this case, is less than 40% (i.e., the node can only support 384 channels, much less than 1024 needed in this example). The only way to scale beyond this limit would be to go through a node expansion where a second layer of 1×32 WSSs is added to every degree, which is complex and costly. Additionally, the CD ROADM is generally lower cost on a per port basis.